Generally, a metal cap obtained by forming of a metal sheet, such as an aluminum-based alloy sheet, a chromate-treated steel sheet, or a tinplate sheet, is by far superior in gas barrier properties to a plastic cap. Moreover, the metal cap can be fixed firmly to a container mouth by threaded engagement. Thus, the metal cap is widely used, for example, as a cap for containers of carbonated beverages. So far, one-piece type metal caps formed by supplying molten resin directly into a cap body, followed by compression molding, to carry out forming and placement of a liner simultaneously within the cap body have been put to practical use.
In the above-mentioned one-piece type metal cap, however, when a tamper evident band is formed, the disadvantage occurs that before bridges ensuring connection of the tamper evident band are broken, the sealing of the cap is released. The problem is also posed that compared with a two-piece type cap, the one-piece type cap requires a high stopper-opening torque, and is inferior in unsealing properties.
As a container closure which solves such problems, a proposal has been made for a container closure including a cap body having a circular top panel wall and a cylindrical skirt wall extending downwardly from the peripheral edge of the top panel wall (i.e., a body made of a metal sheet), a metallic sheet member (a reinforcing plate made of a metal sheet), and a synthetic resin liner (Patent Document 1). In this container closure, the sheet member has a circular wall, the liner is disposed on the lower surface of the circular wall, and an inside plug is constituted by the sheet member and the liner. An inner protruding portion protruding radially inwardly is formed in the skirt wall of the cap body, and a free end edge of the sheet member is engaged with the inner protruding portion, whereby the sheet member is restrained within the cap body, with the circular wall of the sheet member being located opposite the inner surface of the top panel wall of the cap body. The container closure of this configuration is formed by a method which comprises integrating the metallic sheet member with the liner, and inserting the resulting composite into the cap body; or a method which comprises inserting the metallic sheet member into the cap body, then supplying molten resin, and press-forming the resin for formation.